More precisely, this present invention concerns the technical area of sensors which are sensitive to the direction and intensity of a physical magnitude in order to enable detection of the position and even the speed of a mobile target fitted with at least one pointer in the general sense.
More particularly, the subject of the invention concerns the area of magnetic sensors that are adapted in order to enable detection not only of the speed of a mobile target but also its position when the mobile target is in a rest position.
In the current state of these techniques, we are familiar with magnetic sensors of the TPOS type (true power on state) which, when they are switched on, are able to supply a signal corresponding to the position of the mobile target. Such a sensor includes a mobile target created, for example, in a soft magnetic material and presenting at least one and in general a series of teeth separated by gaps. Such a sensor also includes a permanent magnet creating a magnetic gap with the mobile target. In this magnetic gap is placed a probe which is sensitive to the direction and to the intensity of a magnetic induction.
For each passage of a tooth in front of the probe, the movement of the mobile target causes a variation of the magnetic induction passing through the probe, which thus delivers an electrical signal that is a function of the direction and the amplitude of the magnetic induction. This sensitive probe is associated in particular with a hysteresis-type level comparator, whose output takes a first logical state when the electrical signal delivered by the probe is greater than a predetermined threshold and a second logical state when the electrical signal is less than a predetermined threshold.
This type of sensor supplies a signal corresponding to a gap or to a tooth, on switch-on of the sensor, without any target movement. The drawback of such a sensor is its insensitivity to the variation of parameters such as the temperature and the magnetic gap between the target and the sensor. Moreover, the signal delivered by such sensors generally contains a large offset, which renders it difficult to make use of the signal.
In the current state of these techniques, we are also familiar with a position and/or speed sensor that employs two sensitive probes connected to processing resources suitable for processing the output signals of the probes in a differential manner. Such a sensor has the advantage of eliminating the offset of the signal. Moreover, such a sensor is not very sensitive to variations in the magnetic gap and the temperature. However, such a sensor cannot supply a useful signal when switched on, since the differential signal is zero when the probes are located at the centre of a tooth or a gap.
Likewise, U.S. Pat. No. 6,452,381 describes a position sensor that includes a mobile target with gaps and teeth and moving in front of two magnetoresistive probes. The output signals of the probes are processed in a differential manner, so as to increase sensitivity. However the disadvantage of such a sensor is its serious insensitivity to variation of the magnetic gap between the target and the probes.
In the current state of these techniques, we are also familiar, from U.S. Pat. No. 5,444,370, with an angular position sensor that includes a mobile target equipped with two magnetic tracks, each moving in front of a probe which is sensitive to variations in magnetic induction. Although such a sensor is able to supply a useful signal on the position of the target on switch-on of the sensor while the target is not moving, such a sensor also has the drawback of requiring the provision of separate magnetic tracks.
Examination of the current state of these techniques leads to the observation that there is a requirement for a sensor that is practically insensitive to interference effects such as variations in the magnetic gap and the temperature, while still being capable of supplying a useful signal when switched on.
The subject of this present invention therefore aims to meet this requirement by proposing a device to detect at least the position of a mobile target, designed to be practically insensitive to variations of magnetic gap and temperature, while still supplying, at switch-on, a signal corresponding to the position of the mobile target.
In order to attain such an objective, the subject of the invention concerns a device to detect at least the position of a mobile target, equipped with at least one pattern or pointer, and mounted so as to define a magnetic gap in which are placed at least two probes which are sensitive to the intensity of a physical magnitude where, for each passage of a pattern in front of a probe, the movement of the target gives rise to a variation in the intensity of the physical magnitude detected by the probe, which then delivers an electrical output signal that is a function of the amplitude of the physical magnitude, and applied to the input of processing resources. According to the invention, the processing resources include the following:                resources for the determination of a first signal, known as an absolute signal, obtained from at least one output signal,        resources for the determination of a second signal, known as a differential signal, obtained from the difference between at least two output signals,        and resources comparing the absolute signal and the differential signal, delivering a binary signal representing at least the position of the mobile target.        
According to one characteristic of the invention, the processing resources include resources to produce an offset value which is adapted in order to correct either the absolute signal or the differential signal, so that one of the said signals is greater than or less than the other when a pattern is respectively absent from or present in the magnetic gap.
According to one implementation variant, the resources for determination of the absolute signal uses the output signals delivered by the probes in order to calculate their sum.
According to another characteristic of the invention, the processing resources include resources for the inversion either of the differential signal or of the absolute signal so as to obtain a third signal, called the inverse signal.
According to one form of implementation, the comparison resources include a first resource for comparison between the absolute signal and the differential signal (or their inverse signals) so as to detect a passing edge of the target when the values of the said two signals are equal.
According to this form of implementation, the first comparison resource delivers a binary state that is determine when the absolute signal is greater (or less) than the differential signal.
According to another form of implementation, the comparison resources include a second resource for comparison between the absolute signal and the inverse differential signal (or their inverse signals) so as to detect a passing edge of the target when the values of the said two signals are equal.
According to this form of implementation, the second comparison resource delivers a binary state that is determined when the inverse differential signal is greater (or less) than the absolute signal.
Advantageously, the first and second comparison resources are connected to logical processing resources delivering a binary signal at a state which is determined only when the differential and inverse differential signals are less (or greater) than the absolute signal.
The comparison resources can be threshold comparators for example.
By way of an example, the logical processing resources connected to the comparison resources take the form of a logical AND gate.
According to another advantageous characteristic of the invention, the processing resources include a high-pass filter, to the input of which is applied the differential signal with a view to eliminating its offset values.
According to one implementation variant, the absolute signal and the differential signal are determined by assigning to each output signal, gain coefficients which are specified so that the amplitude of the differential signal is equal to or greater than the amplitude of the absolute signal.
Diverse other characteristics emerge from the description provided below, with reference to the appended figures which show, by way of non-limited examples, various forms of implementation of the subject of the invention.